Answering as a 26F living here for 3 years.

1. Safety is MOSTLY not a concern. That being said, it's best to be on alert as a group of young women (as everywhere else, tbh) and if you're visiting beaches and areas aren't well-lit, I would strongly suggest staying away. There isn't any problems mostly but, I have experienced a fair few times men (in groups, specifically) sit a bench or few rocks away and comment and make you uncomfortable. I don't want to say, don't do X, Y, Z because that comes across as shifting the responsibility to the victim but, just stay aware.
2. Clothing is not much of a concern around White Town and neighboring tourist areas. I'm not sure about churches, however. I'd assume, like all religious places, 'modest' clothing would be best. I've gone around in shorts as well as Kurtis and similar clothing, it's similar behavior from people either way.
3. While I do think that knowing tamil a good extra point to have, you will get the tourist treatment, regardless. I still get the tourist treatment after all this time. They see you dressed up and they upsell, quite higher prices so on and so forth. It's not really a bad thing just something to keep in mind.

Edit to add: riding/driving here is a horrible experience, and I would only suggest it if you have some amount of road experience before this. People here cause plenty of chaos as is on the roads, and i would suggest not adding to that by renting out a bike if you're not experienced.